Acoustic filters

ABSTRACT

A body of piezoelectric material propagates acoustic surface waves. Coupled to a surface of the body is a first transducer that launches the waves. Spaced on the same surface from the first device is a second transducer that responds to the launched waves. In order to reduce interference from reflections and increase efficiency, an end surface of the body is selectively located with respect to the adjacent transducer and is formed to define a particular angle with respect to the wave propagating surface. With the transducer elements in the form of interleaved combs of electrodes, absorption of reflections from an end surface is increased by effectively locating the end surface a precise fraction of an acoustic wavelength from the nearest comb electrode or tooth. Moreover, the response of the combined apparatus to signals being transmitted may be altered by changing either the angle that the end surface forms with respect to the propagating surface or by changing the spacing of the end surface from the transducer. The response also may be altered by depositing a different material on the wave propagating surface.

United States Patent n 13,ss1,24s

[72] Inventors Adrian J. De Vries Elmhurst; Thomas J. Wojcik, MountProspect, both of, Ill. [21] Appl. No. 810,774 [22] Filed Mar. 26, 1969[45] Patented May 25, 1971 [73] Assignee Zenith Radio CorporationChicago, Ill.

[54] ACOUSTIC FILTERS 1 Claim, 5 Drawing Figs.

[52] US. Cl 333/72, 3 l0/8.1 [51] Int. Cl ..H04r 17/10, HOlv 7/00 [50]Field of Search 333/30, 70 (S), 72; 3 10/81, 8.2

[56] References Cited UNITED STATES PATENTS 2,886,787 5/1959 Broadheaclet a1 333/72 3,311,854 3/1967 Mason 333/72X 3,376,572 4/1968 Mayo333/72X 3,435,381 3/1969 Tournois 333/30 3,446,975 5/1969 Adler et al.333/72X 3,461,408 8/1969 Onoeetal. 3,464,033 8/1969 Tournois ABSTRACT: Abody of piezoelectric material propagates acoustic surfacewaves. Coupledto a surface of the body is a first transducer that launches the waves.Spaced on the same surface from the first device is a second transducerthat responds to the launched waves. In order to reduce interferencefrom reflections and increase efficiency, an end surface of the body isselectively located with respect to the adjacent transducer and isformed to define a particular angle with respect to the wave propagatingsurface. With the transducer elements in the form of interleaved combsof electrodes, absorption of reflections from an end surface isincreased by effectively locating the end surface a precise fraction ofan acoustic wavelength from the nearest comb electrode or tooth.Moreover, the response of the combined apparatus to signals beingtransmitted may be altered by changing either the angle that the endsurface forms with respect to the propagating surface or by changing thespacing of the end surface from the transducer. The response also may bealtered by depositing a different material on the wave propagatingsurface.

. l ACOUSTIC FILTERS BACKGROUND OF THE INVENTION This invention pertainsto acousto-electric filters. More particularly, it relates tosolid-state tuned circuitry which involves interaction between atransducer coupled to a piezoelectric material and acoustic wavespropagated in that material. The devices have been referred to by theterm SWIF" which is an abbreviation for surface wave integratablefilter.

In copending application Ser. No. 72l,038, filed Apr. I2, 1968, andassigned to the same assignce as the present application, there aredisclosed and claimed a number of different acousto-electric devices inwhich acoustic surface waves propagating in a piezoelectric materialinteract with transducers coupled to the surface waves. In each of thedevices disclosed in that application, the surface waves are launched inthe body of piezoelectric material by a first or transmitting transducerand are caused, in one manner or another, to interact with a secondtransducer spaced along the surface from the first. In the simplestcase, the first or transmitting transducer is coupled to a source ofsignals while the second or receivingtransducer is coupled to a load,the signal energy being translated by the acoustic waves between the twotransducers.

In practice, such devices have been demonstrated to exhibitcharacteristics useable in a number of different applications. In atelevision receiver, for example, acoustic filter systems have beenincluded in the IF channel in order to impose the desired IFcharacteristic with trap or null points at selected frequencies spacedfrom the video IF carrier frequency and determined by the structure ofthe acoustic filters included in the system. As another example, anacoustic filter system may serve in an FM receiver as the discriminatorto perform the necessary function of converting frequency changes toamplitude changes.

While the demonstrations thus far have been highly encouraging, onedifficulty has been encountered that is attributable to the presence onthe propagating surface of reflected waves. These reflected waves'areproduced by the receiving transducer itself and also by a portion of theinitial waves that travel past the receiving transducer to the end ofthe piezoelectric substrate from which they are reflected back along theoriginal path. Additionally, typical forms of transducers launchacoustic waves simultaneously in opposing directions but only one ofthem may be directed toward the receiving transducer. The other wavetravels to an end surface of the piezoelectric substrate and there isreflected or turned back in the opposite direction. In any case, thereflected waves are delayed by a finite time interval so that, whenfinally interacting with the receiving transducer, they develop atimedelayed signal. In atelevision environment, this can result in theappearance of a ghost" in the reproduced image. Also, to the extent thereflected waves do not properly interact with the receiving transducer,the efi'rciency of the device is reduced.

It is, accordingly, one object of the present invention to provide a newand improved acousto-electric filter in which interference fromreflected acoustic surface waves is inhibited.

Another object of the present invention is to provide a new and improvedacousto-electric filter in which undesirable interference from suchreflected waves is inhibited while, at the same time, the reflectedwaves are advantageously utilized.

One reason for the great interest in acousto-electric filters is thefact that they are a solid-state device that may be fabricated byconventional integrated-circuit techniques. In the latter connection,they may be fabricated in combination or together with other active orpassive devices as a single unitary assembly. Once produced, however,the resulting assembly is fixed as to those of its physicalcharacteristics that ordinarily determine such parameters as sensitivityand selectivity.

It is, therefore, a further object of the present invention to provide amethod of altering the frequency response of an acousto-electric filterafter it has been fabricated.

It is still another object of the present invention to provide means foraltering the frequency response of an acousto-electric filter withoutchanging the frequency-determining characteristics of the transducerelements themselves.

SUMMARY OF THE INVENTION The invention is applicable to an acousticfilter having an acoustic-wave-propagating substrate and a surface wavetransducercoupled to a propagating surface of that substrate. Thetransducer interacts with surface waves propagating on a predeterminedportion of the substrate. In accordance with one feature of the presentinvention, an end surface of that substrate, on the side of thetransducer opposite the predetermined portion, forms an angle to thatsurface of a value selected to optimize wave reflection. In accordancewith another feature of the invention, a layer of a material having wavepropagating properties different from those of the substrate, isdisposed on the propagating surface in order to modify its inherentpropagation velocity and thereby alter the frequency response of thefilter. Still another feature of the invention pertains to the spacingof an end surface of the substrate from the transducer such that surfacewaves reflect ed by that end surface combine with surface waves launchedor received by the transducer in a manner to alter the frequencyresponse of the filter.

The features of the present invention which are believed to be novel areset forth with particularity in the appended claims. The organizationand manner of operation of the invention, together with further objectsand advantages thereof, may best be understood by reference to thefollowing descriptiontaken in conjunction with the accompanying drawing,in the several figures of which like reference numerals identify likeelements and in which:

FIG. 1 is a partly schematic plan view of an acoustic filter system;

FIG. 2 is a side-elevational view of a particular acoustic filterstructure;

FIG. 3 is a fragmentary plan view of another acoustic filter system;

FIG. is a plot depicting the response curves of an acoustic filter undertwo different conditions; and

FIG. 5 is a side-elevational view of yet another acoustic filter device.

In FIG. I, a signal source 10 in series with a resistor 11, which mayrepresent the internal impedance of that source, is connected across andimpedance-matched to an input transducer or surface wave interactiondevice 13 mechanically coupled to one major surface of a body ofpiezoelectric material in the form of a substrate 14. An output orsecond portion of the same surface of substrate 14 is, in turn,mechanically coupled to an output transducer 15 which is coupled to aload 18 with matched impedances. Shunted across transducers l3 and 15are respective inductors l6 and 17 that match the transducers to theircoupled input or output stages. These inductors typically are of a valueto resonate with the clamped capacitance of the transducers at theassigned center frequency of the signals to be transmitted. This mode oftuning out the clamped capacitance is further described in theaforenoted copending application.

Transducers l3 and 15 in the simplest arrangement are identical and areconstructed of two comb-type electrode arrays. The stripes or conductiveelements of one comb are interleaved with the stripes of the other. Theelectrodes are of a material such as gold or aluminum which may bevacuum deposited or photoetched on a highly lapped and polished planarsurface of the piezoelectric body. The piezoelectric material is one,such as PZT or quartz, that is propagative of acoustic waves. Thedistance between the centers of two consecutive stripes or teeth in eacharray is one-half of the acoustic wavelength of the signal wave forwhich it is desired to achieve maximum response.

Direct piezoelectric surface wave transduction is accomplished by thespatially periodic interdigital electrodes of transducer 13. Operatingas a transmitter, a periodic electric field is produced when a signalfrom source is fed to the electrodes and, through piezoelectriccoupling, the electric signal is transduced to a traveling acoustic waveon substrate 14. This occurs when the stress components produced by theelectric field in the piezoelectric substrate are substantially matchedto the stress components associated with the surface wave mode. Source10, for example, a portion of a television receiver, produces a range ofIF signal frequencies, but due to the selective nature ofthe arrangementonly a particular signal frequency and its intelligence-carryingsidebands are convertedto a surface wave. More specifically, source 10may be the tunable front end of a television receiver which selects adesired program signal for application to load 18 that, in thisenvironment, comprises those stages of a television receiver thatrespond to the IF program signal in producing a television image and itsassociated audio program. The surface wave resulting in substrate 14 inresponse to the encrgization of transducer 13 by the IF output signalfrom source 10 is translated along the substrate to output transducerwhere it is converted to an electrical output signal for application toload 18.

in a typical television lF embodiment, utilizing PZT as thepiezoelectric substrate, the stripes of both transducer 13 andtransducer 15 are approximately 0.5 mil wide and are separated by 0.5mil for the application of an IF signal in the typical range of 40 46megahertz. The spacing between transducer 13 and transducer 15 is on theorder of 0.05 inch and the width of the wave front is approximately 0.05inch. This structure of transducers 13, 15 and substrate 14 acts as acascaded set of tuned circuits with a resonant frequency ofapproximately 40 megahertz, the resonant frequency being determined, atleast to a first order, by the spacing of the stripes.

The potential developed between any given pair successive stripes inelectrode array 13 produces two waves traveling along the surface ofsubstrate 14, in opposing directions, perpendicular to the stripes forthe illustrative isotropic case of a ceramic poled perpendicularly tothe surface. When the distance between the stripes is one-half theacoustic wavelength of the wave at the desiredinput frequency, or is anintegral multiple thereof, relative maxima of the output wave areproduced by piezoelectric transduction in transducer 15. For increasedselectivity, additional electrode stripes are added to the comb patternsof transducers 13 and 15. Further modifications and adjustments aredescribed in the aforementioned copending application for the purpose ofparticularly shaping the response presented by the filter to thetransmitted signal. Moreover, as described and claimed in copendingapplication Ser. No. 817,093, filed Apr. 17, 1969 in the name of RobertAdler et al. the entire region of substrate 14 need not bepiezoelectric; it is sufficient, and sometimes desirable, to have thepiezoelectric property exhibited only directly under the comb arrays.

As stated above, waves are launched by transducer 13 in opposingdirections. The wave propagated to the right in FlG. 1 travels directlyto transducer 15 where its energy preferably should be entirelytransferred by way of that transducer to load 18. The transducingefficiency, however, less than 100 percent and as a result, one portionof the wave energy is reflected back toward transducer 13, anotherportion is ab sorbed by transducer 15 and a third portion passes throughor beyond transducer 15. it can be shown that, with transducer 15optimally tuned and terminated but without more, these portions areone-fourth, one-half and one-fourth, respectively, of the surface waveenergy launched by transducer 13 in the direction of transducer 15. i

The portion of the wave energy that passes through transducer 15 may bedispersed or redirected so as to avoid a second interaction with thattransducer and consequent confusion of signal information. Serrating theend of the propagating surface beyond transducer 15 will disperse thosewaves, while forming the end of that surface at an acute angle to thedirection of wave propagation will reflect the waves along a differentpath so that they do not again interact with the electrodes of thattransducer. To the extent this portion of the wave energy thus isdissipated, or is otherwise attenuated, it is not used to develop anoutput signal. Similarly, the wave energy that is originally launched tothe left of transducer 13, or directed away from transducer 15, may bedissipated so as not to be reflected for subsequent interaction withtransducer 15 to develop a delayed signal. However, difficulty arisesfrom that portion of the wave energy which is reflected by transducer 15back to transducer 13. Upon rereflection at transducer 13 again totransducer 15, this wave energy develops a delayed signal that is notreadily attenuatable without also suppressing the desired single pathsignal.

To the end of reducing such unwanted reflections and improving overallefficiency, the energy absorption of transducer 15 is increased. This isachieved by causing the substrate end surface beyond transducer 15 to behighly reflecting and by locating that end surface effectively an oddintegral number of surfacc-wavc quarter-wavelengths from transducer 15.The opposite end surface preferably is similarly characterized withreference to input transducer 13. in principle, perfect reflectivityfrom an end surface of the substrate, optimally located with respect tothe adjacent transducer, can increase the efficiency of that transducerby three db and completely eliminate the'reflected energy. in practice,complete reflection and precise location are not obtainable but they maybe approached to such an extent that a significant degree of improvementis found to exist. The basic principles involved find an analogy inradio frequency wave-transmission line practice. When such a line isshorted at a point an odd number of quarter wavelengths beyond itsconnection to a load that presents the characteristic impedance of theline, or is left open a whole number of half-wavelengths beyond theconnection to this load, all of the energy carried by the line isabsorbed into the load. In both cases, all energy is reflected from theend of the line and returned to the load. This teaches that the locationof the load is critical for optimum absorption of incident power intothe'load. In the present environment, the previously described tunedtransducer 15 and its matched load 18 constitute a termination of anacoustic transmission line. The edge of the substrate beyond transducer15 in a direction away from transmitting transducer 13 acts as areflector of acoustic energy traveling beyond transducer 15 andexperimentally it has been found that this edge should be bevelled andshould be located one quarter acoustic wavelength, or an odd multiplethereof, from the center of the nearest stripe of the transducer foroptimum energy transfer to load 18.

in order to create a highly reflective barrier at the end of theacoustic-line, the end surface is bevelled, as stated, or cut to form aparticular angle to the wave propagating surface, namely, the anglewhich for the material of the substrate gives maximum reflection. At thesame time, the end surface is located an odd integral number ofsurface-wave quarterwavelengths from the center of the closest electrodein the associated transducer. Thus, in FIG. 2 the opposing end surfaces23 and 24 of substrate 14, composed of PZT-4, are inclined at an angle aof 50with respect to propagating surface 26 upon which are disposedrespective input and output transducers l3 and 15. That is, it has beendiscovered that the reflection coefficient varies as the angle a ischanged. For PZT4, angle a is 50to obtain an optimum reflectioncoefficient which is estimated to be 95 percent. In contrast, when theedge is formed so that angle a is other conditions being the same, thereflected wave is estimated to have an amplitude of only 50 percent ofthe incident wave.

The arrangement shown in FIG. 3 features an end surface located to actdirectly as an effective part of the transducer itself. As depicted,substrate 14 carries an interleaved-combtype transducing array 15 sodisposed that the adjacent end surface 32 of the substrate is located inthe middle of an elemental transducer that would be formed by twoadjacent stripes 33 and 34 if the substrate and transducer werecontinued or extended outward from the location of that end surface asindicated by broken construction lines. That is, end surface 32, on theside of transducer opposite the active propagating surface 26, iseffectively located midway between the last actual and the next virtualinterdigital tooth. Remembering that the tooth spacing is one-halfwavelength at the frequency of maximum response, end surface 32 thus isspaced a quarter wavelength from the center of last actual tooth on thesubstrate. A wave incident to the transducer first traverses thetransducer is reflected against the bevelled edge 32 and is redirectedto traverse the transducer again. Consequently, it traverses thetransducer twice and therefore the frequency response of a transducerwith a bevelled e'dge located for optimum power transfer is equivalentto that of a transducer with twice the actual number of stripes. This,incidentally, narrows the frequency response.

When filters of the type described are composed of materials in whichthe sound velocity varies somewhat from one batch to the next, it isdesirable to be able to control the frequency of maximum response afterthe filter has been fabricated and the transducer electrode spacingsthus have been fixed. That is, as fabricated, the filter may initiallyexhibit a frequency response curve 37 as shown in FIG. 4. Either tocorrect a variation in sound velocity or to allow tuning in a particularsystem, it may be desired to alter the shape or position of that curveso as generally to occupy a higher frequency range as indicated by curve38 in FIG. 4. To achieve such a shift in the frequency of maximumresponse, the location of end surface 32 relative to transducer array 15is changed as by cutting or grinding the end surface so as to move it,in effect, slightly toward the transducer. Recalling that the reflectingedge or end surface 32 acts like a mirror in that it establishes avirtual image of the actual transducer, shifting the position of thatreflective surface changes the average interelectrode or tooth spacingof the combined virtual and actual transducers as viewed in terms ofinteraction of both the originally created and reflected'waves. The sameeffect is obtained in the filter of FIG. 2 by removing material from theend surface adjacent to the transducer the response of which is to bealtered. In all of these cases, the frequency of maximum response isshifted by an amount which in practice is limited to the order of 50 INpercent, where N is the number of electrodes or teeth in the associatedtransducer; this limitation exists in practice because only a fractionof a wavelength can be removed in any effort to change the responsefrequency. The frequency response likewise may be changed in the filterof FIG. 2 by grinding or cutting away from end surfaces 23 or 24, orboth,

in order to change the value of angle a. In general, the effect is tobroaden the response by reducing the effective number of virtual stripesthat are added.

Further in connection with the use of slanted or bevelled end surfacesas in the filter of FIG. 2, it may be noted that the effective locationof those end surfaces with respect to their respective transducers hasbeen found to be slightly different than the physical distance from thecenter of the outermost transducer electrode to the outer edge of wavepropagating surface 26 )(the apex of angle a). That is, the effectivelocation of the end surface appears to be along a line situated slightlytoward the associated transducer from the outermost edge of thepropagating surface.

As discussed, fabrication of the acoustic filter in accordance with apredetermined pattern and spacing of the interdigital teeth yields adevice that exhibits a maximum response at a selected frequency. Thatfrequency is primarily a function of the interelectrode spacing inrelation to the velocity of propagation of the surface waves along thesubstrate. In FIG. 5, the acoustic filter includes a substrate [4 havinga wave propagation surface 26 along which are disposed respective inputand output transducing electrode array I3 and 15. Affixed directly topropagating surface 26, particularly in the transducer regions, is alayer of a nonconductive material 44 that has an effectivewave-propagation velocity different from that of surface 26. Inpractice, this may be an actual difference in any one or more ofspecific wave velocities, masses or other acoustic properties. In oneexemplary embodiment operating at seven megahertz, material 44 simply isa spray-type lacquer having'a thickness on the order of 0.005 inch.Quick drying spray enamels also are conveniently employed. By suchtechniques, the effective surface wave velocity, and hence the frequencyof maximum response, can readily be changed in the order of 1 percent.With that degree of correction, a slight increase in attenuation of thetransmitted signals occurs, but this is generally of minor significance.

The foregoing techniques and structures impart and advantageous degreeof flexibility in the fabrication and use of acousto-electric filters.The impairment in performance which may result by reason of the presenceof undesired reflected waves can be obviated either by inhibiting or byactually making use of those reflected waves. In addition, structuraland procedural approaches have been disclosed which enable a useabledegree of tuning for adjustment of the frequency of maximum response ofthe filter. These approaches are beneficial either for the purpose ofcorrecting manufacturing variations or of permitting adjustment of thefilter frequency with respect to operation of the system in which thefilter is utilized.

While particular embodiments of the present invention have bee shown anddescribed, it will be obvious to those skilled in the art that changesand modifications may be made without departing from the invention inits broader aspects. Accordingly, the aim in the appended claims is tocover all such changes and modifications as fall within the true spiritand scope of the invention.

We claim:

1. In an acoustic filter having an acoustic-wave-propagating substrateand a surface-wave transducer actively coupled to a propagating surfaceof that substrate for interacting with surface waves propagating on apredetermined portion of the substrate, said transducer including anarray of interleaved combs of conductive electrodes defining a patternof spaced interdigital teeth, the improvement in which the end surfaceof said substrate on the side of said transducer opposite saidpredetermined portion is effectively located midway between an actualone of said teeth and an adjacent virtual image of said one tooth.

1. In an acoustic filter having an acoustic-wave-propagating substrateand a surface-wave transducer actively coupled to a propagating surfaceof that substrate for interacting with surface waves propagating on apredetermined portion of the substrate, said transducer including anarray of interleaved combs of conductive electrodes defining a patternof spaced interdigital teeth, the improvement in which the end surfaceof said substrate on the side of said transducer opposite saidpredetermined portion is effectively located midway between an actualone of said teeth and an adjacent virtual image of said one tooth.